The frequency of a grid, particularly a large or “infinite” grid is relatively stable. All synchronous generators on the grid will synchronize to the grid frequency. Load on the grid is inversely proportional to grid frequency, so that as the load increases relative to the combined generator output, grid frequency decreases.
Droop frequency control is used in such synchronous generators to control power output. When a reference frequency is above a grid frequency, power is added to the prime move, e.g., more fuel is provided to an engine driving the generator, so that torque is increased and more power is output. Droop control systems use a linear function around the desired power level to manage the frequency of the output as a function of power.
Statcom systems are used in the utility power grid to provide reactive power compensation and transient voltage support. Statcom systems are generally capacitor banks and have very short duration operational times, in the order of seconds. U.S. Pat. No. 7,508,173 teaches use of an inverter to provide reactive power in a power distribution network.
In a microgrid application, a wide variety of power generation sources may contribute to a particular grid, including solar panels, wind turbines, diesel or gas generator sets, fuel cells, and even the utility grid. In this environment, power generation levels, frequency, and voltage may vary in a wide range due to the volatile nature of some of the power sources and sluggish response times of generator sets. Statcom systems may not have the capacity for grid support over the longer durations that may be occur in micro grid applications. In these cases, energy storage systems, using batteries, ultra capacitors, etc., may be used to provide supplemental power. However, energy storage systems (ESS) present additional requirements for long-term charge maintenance and ESS component protection.